The present invention relates to a process for producing liquids containing hydrocarbons of a higher molecular weight than methane from a hydrocarbon feedstock containing a major proportion of methane.
Gas-to-liquid conversion technologies use chemical means to convert methane or natural gas to a liquid form suitable for ready transport or direct use. This conversion is accomplished by altering the composition of the hydrocarbon gas molecules to form stable liquids that can be used directly as a chemical feedstock or transportation fuel. There are two known approaches to accomplish this conversion; partial oxidation and oxidative coupling. In the partial oxidation process, hydrocarbons, oxygen and/or water are converted to a synthetic gas containing molecular hydrogen and carbon monoxide. These constituents are recombined in a second process to produce paraffins and high molecular weight fuels such as diesel fuel and heating oil. In the oxidative coupling reaction, hydrocarbon gases are directly converted into desirable liquid hydrocarbons through a series of free radical addition mechanisms.
Known oxidative coupling technologies use oxygen to convert methane to the methyl free radical and water in the presence of a catalyst at temperatures of 800 to 1000xc2x0 C. The major challenge of these technologies is the rapid conversion of the radicals to carbon dioxide before the radicals can link-up, greatly limiting the conversion to higher molecular weight compounds.
Previous synthetic routes to producing higher molecular weight hydrocarbons from lower molecular weight hydrocarbons have started from feedstocks which have at least two carbon atoms. Such feedstocks are initially dimerised or oligomerised at temperatures in the region of 500-600xc2x0 C. Such processes are described, for example, in U.S. Pat. Nos. 1,677,363; 4,721,828; 4,769,828; and 5,414,176.